<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .box {
      width:80%;
      height: 200px;
      margin: 100px auto;
    }  
    .box div{
      float: left;
      height:100%;
      width:33.33%
    }
    .box div:nth-child(1){
      background-color: red;
    }
    .box div:nth-child(2){
      background-color: green;
    }
    .box div:nth-child(3){
      background-color: blue;
    }

  </style>
</head>
<body>
  <div class="box">
    <div></div>
    <div></div>
    <div></div>
  </div>
</body>
</html>